Method for detecting an attempted attack, recording medium, and security processor for said method

ABSTRACT

This method in which an attempt to attack a security processor is detected by the security processor itself comprises:
         measurements ( 50 ) of several different events occurring independently of one another in the absence of attack attempts,   building ( 52 ) the value of at least one attack indicator as a function of at least one index of concomitance between at least two different events measured, the index of concomitance representing the temporal proximity between the two different events measured, and   detecting ( 54 ) an attack attempt if the value of the attack indicator crosses a predetermined threshold.

The invention pertains to a method by which an attempt to attack a security processor is detected by the security processor itself. An object of the invention is also an information-recording medium as well as a security processor to implement this method.

Security processors are generally hardware components containing confidential information such as cryptographic keys or access rights which only legitimate users can use. To preserve the confidentiality of this information, these processors are designed to be as robust as possible against attack attempts by computer hackers. For example, a security processor is a chip card equipped with an electronic processor.

Security processors are subjected to different types of attack. Some of these attacks are aimed at extracting or determining the confidential information contained in the security processor. To this end, a multitude of attacks have been developed. For example, certain of these attacks seek to obtain an abnormal functioning of the security processor by making it process messages built by computer hackers. Other more invasive methods try to disturb the functioning of the security processor at key moments in its operation by playing on its supply voltage or again by means of a laser beam directed towards the security processor.

Other types of attack do not seek to extract or determine the confidential information contained in the security processor but consist simply of the abusive use of this security processor. For example, in pay television, control sharing and card sharing come under this type of attack. To put it briefly, control sharing consists of the sharing of the control word deciphered by the security processor amongst several receivers. These receivers can then decipher the scrambled multimedia contents with this control word whereas the subscription has been paid for by only one receiver.

In card-sharing a same security processor is made to decipher several enciphered control words coming from different receivers. As above, all these receivers can then descramble the scrambled multimedia contents whereas only one of these receivers is entitled to access the content.

To combat these attacks, there are known ways of detecting attack attempts and, in response to this detection, to execute countermeasures.

One example of a method for detecting attack attempts and for executing countermeasures in response is described for example in the patent application EP 1 575 293.

A countermeasure is an action aimed at preventing an attack against the security processor from being long-lasting or successful. There are a large number of countermeasures that can be executed by a security processor. These measures range from a simple increase in security measures in the security processor up to the definitive and irreparable blocking of the security processor which then becomes unusable.

Methods for detecting an attack attempt have already been proposed. These methods comprise:

-   -   measuring several different events occurring independently of         one another in the absence of attack attempts, then     -   comparing each measurement with a predetermined respective         threshold to detect the presence or absence of an attack         attempt.

However, the difficulty comes from the fact that the events representing an attack attempt can also occur when there is no attack attempt. Now, it is necessary to prevent the production of false detections of attack attempts because these may result in the untimely execution of countermeasures which then inconvenience the legitimate user of the security processor. For this reason, there are known ways of choosing a far higher value for the predetermined threshold than the values of all the measurements that can be obtained when there are no attacks. However, the choice of a high predetermined threshold makes certain attacks undetectable or slows down the detection of an attack attempt.

The invention seeks to overcome this problem by proposing a method for detecting an attack attempt which also comprises:

building a value of at least one attack indicator as a function of at least one index of concomitance between at least two different events measured, the index of concomitance representing the temporal proximity between the two different events measured, and

detecting an attack attempt if the value of the attack indicator crosses a predetermined threshold.

The above method takes account of the temporal proximity between different events occurring in the security processor. This enables the swifter detection of an attack attempt or the detection of an attack attempt that could not be detected by observation of the measurement of a single event. Indeed, it can happen that the measurement of each of these events, taken individually, does not constitute a speedy representation of an attack attempt since these events occur during a normal operation of the security processor. On the contrary, when these events occur almost concomitantly whereas they should normally occur independently of one another, it can be taken to mean, with a high degree of confidence, that an attack attempt has taken place. The above method therefore enables the security processor to make a swift detection, with a high degree of confidence, of detect the fact that it has been the victim of an attack attempt. The execution of appropriate countermeasures can then be activated with greater speed.

The embodiments of this method may comprise one or more of the following characteristics:

-   -   the value of the attack indicator is built out of several         indices of concomitance between different events measured and by         weighting the importance of these indices of concomitance         relatively to one another by means of a predetermined set of         weighting coefficients;     -   the method comprises the building of several attack indicator         values through the use of several different sets of weighting         coefficients between the same indices of concomitance, each set         of weighting coefficients being predetermined so as to be more         sensitive to an attack attempt that is different from the ones         to which the other indicators are more sensitive;     -   the weighting coefficient of a same index of concomitance is the         same in all the sets of weighting coefficients used to build the         different attack indicator values;     -   the measurement of an event is limited to a sliding time slot so         as to not take account of events have occurred outside this time         slot, this being done so to establish the fact that these events         are concomitant in the measurement of this time slot;     -   at least one of the events measured is the detection of an error         in the functioning of the security processor, each occurrence of         which leads the security processor to stop the processing         operations in progress and to get automatically reset to resume         these operations from the start;     -   the measurement of an event lies in the counting, in a counter,         of the number of times that this event has occurred, the value         of the counter constituting the measurement;     -   the index of concomitance between at least two measurements is         obtained by multiplying these measurements with one another. I

These embodiments of this method furthermore have the following advantages:

-   -   the use of weighting coefficients between the indices of         concomitance simply modifies the sensitivity of the attack         indicator built to a particular type of attack by modifying the         value of these weighting coefficients,     -   the use of several different sets of weighting coefficients         makes it possible, in using the same set of indices of         concomitance, to build several attack indicators each dedicated         to the detection of a different attack attempt,     -   the systematic use of the same weighting coefficient for the         same index of concomitance limits the quantity of memory         required to store these different weighting coefficients,     -   limiting the measurement of an event to a sliding time slot         limits the number of false detections of attack attempts caused         by the accumulation in time of measured events when there are no         attack attempts,     -   when one of the measured events is the detection of an error of         the security processor causing this security processor to be         reset, then the security is increased through prevention of the         untimely blocking of this security processor.

An object of the invention is also an information-recording medium comprising instructions for executing the above method, when these instructions are executed by an electronic computer.

Finally, an object of the invention is also a security processor comprising:

registers in which there are stored measurements of several events occurring independently of one another in the absence of attack attempts, and

a computer capable of

-   -   building the value of at least one attack indicator as a         function of at least one index of concomitance between at least         two different events measured, the measurements of which are         stored in the registers, this index of concomitance representing         the temporal proximity between the two different events         measured, and     -   detecting an attack attempt if the value of the attack indicator         crosses a predetermined threshold.

The invention will be understood more clearly from the following description, given purely by way of a non-restrictive example and made with reference to the appended drawings of which:

FIG. 1 is a schematic illustration of a system for transmitting scrambled multimedia contents that comprises a security processor,

FIG. 2 is a schematic illustration of a matrix of weighting coefficients used by the security processor of the system of FIG. 1,

FIG. 3 is a schematic illustration of a table of warning thresholds used by the security processor of FIG. 1, and

FIG. 4 is a flowchart of a method for detecting an attack attempt on the security processor of the system of FIG. 1.

In these figures, the same references are used to designate the same elements.

Here below in this description, the characteristics and functions well known to those skilled in the art shall not be described in detail. Furthermore, the terminology used is that of systems ofr conditional access to multimedia contents. For more information on this terminology, the reader may refer to:

“Functional model of a conditional access system” EBU Review—Technical European Broadcasting Union, Brussels, BE, No 266, 21 Dec. 1995.

FIG. 1 represents a system for subscriber broadcasting of multimedia contents. For example, the system 2 is a system for broadcasting several scrambled television channels. The descrambling of each of these television channels or groups of television channels is conditional upon the payment of a subscription by subscribers. In this description, the terms “scramble”/“encipher” and “descramble”/“decipher” are considered to be synonymous.

The system 2 has at least one transmitter 4 of scrambled multimedia contents and a multitude of receivers capable of descrambling the multimedia content broadcast by the transmitter 4. To simplify FIG. 1, only one receiver 6 has been shown. For example, the other receivers are identical to the receiver 6.

The receiver 6 is connected to the transmitter 4 by means of a long-distance information-transmitting network 8. The network 8 may be a wireless communications network or a wired network such as the Internet.

Typically, the transmitter 4 broadcasts multiplexed scrambled multimedia contents with ECM (Entitlement Control Message) and EMM (Entitlement Management Message) type control messages. Each ECM message comprises at least one cryptogram CW* of a control word CW used to descramble the scrambled multimedia content.

The receiver 6 has a decoder 10 and a security processor 12 connected detachably to the decoder 10.

The decoder 10 has a receiver 14 of the data transmitted by the transmitter 4 connected to a demultiplexer 16. The demultiplexer 16 demultiplexes the data received and transmits the scrambled multimedia content to a descrambler 18 and the ECM or EMM messages to the security processor 12.

The processor 12 receives the cryptogram CW* and deciphers this cryptogram in order to send the control word CW in unencrypted form to the descrambler 18. This deciphering is permitted only if the access rights contained in the ECM correspond to the access rights stored in the security processor 12. For example, the processor 12 is the processor of a chip card.

The descrambler 18 descrambles the scrambled multimedia content by means of the control word CW deciphered by the security processor 12. The descrambled multimedia content is then for example displayed in unencrypted form on a screen 20 so that the displayed multimedia content is directly perceptible and comprehensible to the user.

The functions and characteristics of the processor 12 for performing the different operations related to the deciphering of the control word CW are known and shall not be described herein in greater detail.

The processor 12 has an electronic computer 24 connected to sensors 26, 27 and a set 30 of registers.

The sensor 26 has a voltage transducer capable of converting the power voltage of the processor 12 into a piece of digital data that can be exploited by the computer 24.

The sensor 27 comprises a light transducer capable of converting the photons of a laser beam directed to the processor 12 into digital data that can be exploited by the computer 24.

By way of an illustration, the set 30 comprises eleven registers referenced C₀ to C₁₀. Each of the registers C₁ to C₁₀ is designed to contain a measurement of an event which may be activated by an attack attempt on the processor 12. The measured events may also occur in the absence of attack attempts. However, when there is no attack attempt, these measured events occur independently of one another. Thus, it is improbable that the measured events will occur concomitantly when there is no attack attempt. The term “concomitantly” designates the fact that these events occur during a same time slot. Here, one time slot is associated with each measured event. This time slot may have a finite duration or on the contrary an infinite duration. In the case of a finite duration it means that the events which occur outside this time slot are not taken into account in the measurement of this event. Here, a time slot of finite duration is a sliding time slot. This sliding time slot has a finite duration which is shifted as and when time elapses, so that only the most recent events are taken into account for the measuring of this event. An infinite duration means that all events, starting with the time of activation of the measurement of this event, are taken into account for the measurement.

Here, the measurement of an event consists in counting the number of times in which this event has occurred during the time slot associated with this event. Thus, each of the registers contains a number representing the number of occurrences of a same event. Consequently, here below in the description, the registers C_(i) are called counters C_(i).

There are a large number of measurable events. Typically, the measured events come under one of the following categories:

-   -   normal events that a legitimate user can activate but which, if         they occur in large numbers, represent an abnormal use of the         processor 12,     -   the reception by the processor 12 of erroneous or unnecessary         images, i.e. messages which do not exist during normal and         error-free use,     -   the detection of errors of functioning of the processor 12.

There are many errors of functioning. For example, the errors of functioning may be errors in the execution of the code of the operating system of the processor 12, an abnormal situation measured by the sensors 26 or 27, errors of integrity discovered in the data processed, etc. In general, in the event of a detection of operating errors, the processing operations in progress are interrupted and the processor 12 gets automatically reset.

An example of events measured for each counter shall now be described in detail.

The counter C₁ contains the number of times in which a command for consulting data from the processor 12 has been received. Indeed, a certain number of pieces of data contained in the processor 12 can be freely consulted. For example, there are commands for consulting the identification number of the processor 12 or access rights recorded in the processor 12. The reception of a consultation command is therefore a normal event so long as it remains occasional. However, the counting of a large number of commands for consulting data in the processor 12 within a short period of time may be caused by an attack attempt.

The counter C₂ indicates a presence of unusual rights recorded in the processor 12. An unusual right is a right which the operator of the system 12 does not normally use. For example, most operators never record a right in the security processors for which the duration of validity is greater than one year. This means that a right recorded in the processor 12 with a duration of validity greater than one year, for example a right with an infinite duration of validity, is an unusual right even if this possibility is technically provided for. Similarly, normally the operator never records a right authorizing access to and deciphering of all the multimedia contents in the security processors. Thus, the registering of a right permitting access to all the multimedia contents in the processor 12 is considered to be an unusual right. The recording of an unusual right in the processor 12 may come from an error by the operator but may also represent an attack attempt.

The counter C₃ counts a number of messages received by the processor 12 that have no functional utility for the processor. For example, such a message with no functional utility may be:

-   -   a message for consulting data that is non-existent in the         processor,     -   a message for erasing a non-existent piece of data (for example         access code etc) in the processor, or     -   two successive messages for reading the same piece of data in         the processor 12.

These messages are syntactically correct and do not prompt any error of execution in the processor 12. However, they are unnecessary. Such unnecessary messages may be sent erroneously by the operator. They may also be used during an attack attempt.

The counter C₄ counts the number of syntax errors in the messages transmitted to the processor 12, i.e. in the ECM and EMM messages transmitted to this processor. Indeed, the syntax or structure of the ECM and EMM messages complies with a predetermined grammar. The processor 12 can therefore detect these errors of syntax and count them in the counter C₄. The errors of syntax can be caused by an error of the operator but also during an attack attempt.

The counter C₅ is a counter of replayed commands whereas they should normally not have to be replayed several times. The replaying of a command consists in sending the security processor the same command several times. For example, the command may be an updating message for updating certain pieces of data recorded in the processor 12. A replay of a message can be detected by the processor 12 by recording the date of the last updating message.

The counter C₆ counts the number of integrity errors detected in the messages received by the processor 12. Indeed, the messages received by the processor 12 contain data as well as a cryptographic redundancy of these pieces of data, enabling the processor 12 to check that there is no error in the data received. For example, redundancy in data may be obtained by integrating a signature or a CRC (Cyclic Redundancy Check) of the data contained in this message. Errors in the data contained in the message may be prompted by disturbances when they are being transported in the network 8 or in the decoder 10. However, erroneous data are also used during an attack attempt.

The counter C₇ counts the number of integrity errors in the pieces of data contained in the processor 12. Indeed, a certain number of pieces of data recorded in the processor 12 are associated with a cryptographic redundancy used to check the integrity of the respectively recorded pieces of data. Once again, it can happen accidentally following for example electromagnetic disturbances that a piece of data recorded in the processor 12 will be erroneous. However, the presence of erroneous data recorded in the processor 12 can also represent an attack attempt.

The counter C₈ counts the number of bad branches during the execution of the execution of the code of the operating system of the processor 12. A bad branch is an untimely or erroneous jump in an instruction executed by the processor 12 to another instruction. These bad branches in the execution of the code may be detected by executing the same instructions on the same pieces of data twice in succession. If the two executions of the code do not give the same result, this means that there has been a bad branch. Untimely jumps from instructions in the code executed by the processor 12 may be prompted by playing on the supply voltage of the processor 12 or directing a laser beam toward this processor 12.

The counter C₉ counts the number of times that the data retrieval procedure is executed after the processor 12 has been wrenched out. The wrenching out of the processor 12 consists of the removal of the processor 12 from the decoder 10 during operation so that the power supply to the processor 12 is interrupted during data processing. The data retrieval procedure makes it possible, after such a power cut, to return the processor 12 to the state in which it had been before the power cut. The processor 12 can be accidently wrenched out of the decoder 10. However, untimely cuts in power supply to the processor 12 are also frequently used during an attack attempt to prevent the execution of countermeasures by the processer 12.

The counter C₁₀ counts the number of times that the abnormal power supply is measured by the sensor 26 totalled up with the number of times that a laser beam is detected by the sensor 27. Indeed, abnormal voltages as well as the presence of a laser beam are typical of an attack attempt on the processor 12. However, these sensors 26 and 27 can also detect abnormal voltage or the presence of a laser beam accidentally following for example electromagnetic disturbances caused by an apparatus in the vicinity of the processor 12, even when there is no attack attempt. For example, the powering on of the decoder 12 can result in the detection of an abnormal voltage by the sensor 26.

The counter C₀ must be distinguished from the previous counters because it counts an event which that occurs only during normal operation of the processor 12 and cannot be caused by an attack attempt. For example, the event counted by the counter C₀ here is the number of ECM and EMM messages properly processed by the processor 12.

The value of this counter C₀ is used to limit the temporal memory of certain previous counters to a sliding time slot with finite duration. For example, to this end, the value of the counter C₀ is subtracted from the value of the counter C_(i), where i>0, and only the difference between these two counters, brought to 0 if it is negative, is used to compute an index of concomitance as described further below. For example here, except for the value of the counters C₈ and C₁₀, only the difference between the values of the counters C_(i) and C₀ is used to compute indices of concomitance. Through the use of the value of the counter C₀, the events that have occurred outside the sliding time slot thus defined are not taken into account to detect an attack attempt. It will be noted that the duration of the sliding time slot defined by means of the counter C₀ is not constant and depends on the use made of the processor 12.

The computer 24 is connected to a memory 32 containing the different pieces of data and instructions needed for the functioning of the processor 12. In particular, the memory 12 comprises:

-   -   instructions needed to compute the method of FIG. 4 when they         are executed by the computer 24,     -   a matrix 36 of weighting coefficients and a table 38 of warning         thresholds.

An example of a matrix 36 is represented in greater detail in FIG. 2. This matrix 36 contains as many pieces of data as there are event counters liable to be activated by an attack attempt. Here, the matrix 36 is therefore a matrix with 10 columns each associated with a counter C_(i). The matrix 36 also contains nine rows associated respectively with the counters C₂ to C₁₀.

The cell situated at the intersection of the ith column from the left and the jth row from the top contains a weighting coefficient m_(i,j) associated with an index of concomitance C_(i)C_(j+1). An index of concomitance C_(i)C_(j+1) is an index computed from the value of the counters C_(i) and C_(j+1) which gives an indication on the concomitance between the events counted by the counter C_(i) and those counted by the counter C_(j+1). Here, each index of concomitance is built so that its value is all the higher as a large number of events measured respectively by the sensors C_(i) and C_(i+1) have occurred in proximity at the same instant. To this end, in this embodiment, each index of concomitance C_(i)C_(j+1) corresponds to the product of the values of the counters C_(i) and C_(j+1) at the same point in time.

The table of warning thresholds 38 illustrated in FIG. 3 comprises a first column containing four warning thresholds S₁ to S₄. Each warning threshold is a numerical value and these warning thresholds are classified in rising order from top to bottom in the table 38.

The table 38 also has a second column associating one or more countermeasures denoted as CM_(i) with each threshold S_(i). The countermeasures are actions executed by the security processor 12 which are designed to make it more difficult to extract or determine data contained in the processor 12 or wrongfully use this processor 12.

Here, the countermeasures CM_(i) associated with the threshold S_(i) are less strict and entail fewer penalties for the user of the processor than those associated with the higher warning threshold S_(i+1). Thus, the higher the warning threshold S_(i) crossed, the stricter will be the countermeasures CM_(i) executed in response.

By way of an illustration, the countermeasures CM₁ consist of the adding of a redundancy to the additional branching of the code to be executed by the processor 12. For example, this redundancy is obtained by executing the conditional branch several times and checking that the result obtained is the same at each execution.

The counter measure CM₁ consists in furthermore adding redundancy to the operations for checking the integrity of the processed data. For example the integrity of the data is checked several times whereas, if the threshold S₁ is not crossed, it is verified only once. It also consists in checking the integrity of the data whose integrity is not checked if the threshold S₁ is not crossed.

The countermeasures CM₂ consist for example in adding restrictions to the possibilities of stringing instructions of the code executed by the processor 12. This can be obtained by forcing the processor 12 to execute a full block of instructions without allowing for any interruption between the execution of the instructions of this block.

The countermeasure CM₂ also consists in eliminating certain functions of the processor 12 hitherto permitted when the threshold S₂ was not crossed. For example, the addition of new services such as the addition of a new operator or a new subscriber is prohibited. The access to the administrative functions of the processor 12 can also be prohibited if the threshold S₂ is crossed.

For example, the countermeasures CM₃ consist in modifying the weighting coefficients present in the matrix 36 so that the upper threshold, i.e. S₄ is easily and speedily reached when events are measured. Thus, as described with reference to FIG. 4, the sensitivity of the processor 12 to the detection of an attack attempt is increased. The countermeasures CM₃ also include the systematic and duplicate checking of the integrity of each message received. The countermeasure CM₃ can also consist in boosting controls on the execution flow. This may especially consist getting each portion of an executable code to be executed twice by the processor 12 and in checking, by comparison at the end of these two executions, that the results obtained are the same. In the event of discrepancy between the results obtained, the counter C₈ is incremented.

Finally, the countermeasures CM₄ definitively invalidate the processor 12 so that it is definitively unusable. For example, to this end, the confidential information contained in the processor 12 is erased.

The functioning of the processor 12 shall now be described in greater detail with reference to the method of FIG. 4.

Along with the normal operation of the processor 12, this processor also executes a method for detecting an attack attempt. To this end, at a step 50, it measures events likely to have been caused by an attack attempt. Here, this measurement consists in counting the corresponding event in the counters C.

Then, during a step 52, the processor 12 builds three attack indicators, I₁, I₂ and I₃ respectively.

The indicator I₁ is conceived so as to be more sensitive to attack attempts using laser disturbance than the indicators I₂ and I₃. An attack by laser disturbance consists in pointing a laser beam to the security processor to prompt instruction jumps in the code executed by this processor at key moments in its execution. The key moments typically correspond to conditional branches.

Here, the value of the indicator I₁ is given by the following relationship:

I ₁ =m _(2,6) C ₂ C ₇ +m _(2,7) C ₂ C ₈ +m _(2,9) C ₂ C ₁₀ +m _(7,7) C ₇ C ₈ +m _(7,9) C ₇ C ₁₀ +m _(8,9) C ₈ C ₁₀

where:

-   -   m_(i,j) is the weighting coefficient, the value of which is         contained in the matrix 36.

The indicator I₂ is designed to be more sensitive to logic attacks than the other two indicators. A logic attack consists in making a search for a logical flaw or an error of implementation in the code executed by the processor 12 so as to obtain an abnormal behavior in this processor. For example, the logic attack consists in sending a very large number of erroneous messages to the processor 12 which are all different from one another until one of these messages prompts an abnormal behavior in the processor 12.

For example, the value of the indicator I₂ is built by means of the following relationship:

I ₂ =m _(1,2) C ₁ C ₃ +m _(1,3) C ₁ C ₄ +m _(3,3) C ₃ C ₄

Finally, the indicator I₃ is designed to be more sensitive to DPA (Differential Power Analysis) attack attempts. A DPA attack is an attack in which a large number of messages are sent to the processor 12 to prompt a large number of executions of cryptographic algorithms on a large number of different pieces of data and at the same time the consumption of current of the processor 12 is measured. Then, through a statistical analysis on the data collected, it is possible to discover the values of the keys or confidential data recorded in the processor 12.

For example, the indicator I₃ is built by means of the following relationship:

I ₃ =m _(4,4) C ₄ C ₅ +m _(4,5) C ₄ C ₆ +m _(5,5) C ₅ C ₆.

Then, once the value of the indicators I₁ to I₃ has been built, in a step 54, the value of these indicators is compared with the different warning thresholds recorded in the table 38 to detect an attack attempt.

If none of these indicators has had its value cross the threshold Si then, in a step 56, no countermeasure is executed.

Conversely, if the value of one of these indicators crosses one of the thresholds S_(i), then a countermeasure associated with the highest crossed threshold I S_(i) is executed in a step 58.

At the end of the steps 56 and 58, the method returns to the step 50.

At the same time as the steps 50 to 58, during a step 60, the transmitter 4, using for example an EMM or ECM type message, transmits new values for the weighting coefficients. Then, in a step 62, the processor 12 receives this message and updates the values of the weighting coefficients contained within the matrix 36.

The updating of the weighting coefficients makes it possible to easily modify the sensitivity of an indicator to a particular attack attempt. In particular, it may be noted that, to modify this sensitivity of the indicator to a particular attack attempt, that is necessary only to modify the weighting coefficients recorded in the matrix 36 without its being necessary to modify other instructions executable by the processor 12.

Many other embodiments are possible. For example, the processor 12 can include several different matrices of weighting coefficients. Each of these matrices can be used to compute a respective attack indicator. This then makes it possible to assign each index of concomitance a different weighting coefficient as a function of the attack indicator built. The use of several weighting coefficient matrices can also be useful for modifying the weighting matrix used when a new warning threshold is crossed.

Conversely, a single attack indicator can be built instead of several indicators.

As a variant, there are as many warning threshold tables as there are attack indicators built. In this variant, the warning thresholds associated with one particular attack indicator are not necessarily the same as the warning thresholds associated with another attack indicator.

The table 38 can also be replaced by a single warning threshold associated with countermeasures.

One of the counters C_(i) can simply account for the existence of an event without counting the number of occurrences of this event. In this case, the value of this counter is encodable by means of a single information bit. Even in this case, the value of the existence counter can be associated with a time slot of infinite or finite duration.

Sensors other than those described can be implemented in the processor 12. For example, the processor 12 can also include a temperature sensor.

The index of concomitance is not limited to the product of two measurements. For example, it may also correspond to a product of more than two measurements. However, the increase in the number of measurements multiplied together also increases the size of the matrix of weighting coefficients.

It is also possible to compute an index of concomitance representing the time proximity between at least two events by mathematical operations other than a multiplication.

Many countermeasures other than those indicated here above can be executed in response to the crossing of a warning threshold by one of the attack indicators. For example, other countermeasures may consist in modifying the cryptographic algorithm executed by the processor 12. A countermeasure can also consist of the use of or the measurement of many events to build an attack indicator which hitherto was not measured. For example, in response to the crossing of a warning threshold, the events measured by one of the sensors 26 or 27 can be counted whereas they were not counted previously.

The computer 24 can be formed by one or more processors. For example, it can be formed by a processor with which there is an associated co-processor. The method of detection can then be executed both by the processor and by the co-processor.

The measurement has been described here in the particular case where the number of occurrences of an event has to be counted. However, the measurement can also consist of the recording, in one of the registers, of the value of an event such as for example a value measured by one of the sensors 26 or 27. To count the number of occurrences of an event, the value of the counters can also be decremented instead of being incremented as described here above.

The architecture of the receiver 6 is herein illustrative solely of a particular situation. In particular, the descrambler 18 can also be detachable. Conversely, the descrambler and the security processor can be implemented without any degree of freedom in the decoder. In this case, the descrambler and the security processor can take the form of software components. 

1-9. (canceled)
 10. A method for detecting, by a security processor, an attempt to attack said security processor, said method comprising measuring events occurring independently of each other in absence of attack attempts; obtaining at least one index of concomitance between at least two measurements by multiplying said measurements by one another, said at least one index of concomitance representing a temporal proximity between at least two different measured events, building a value of at least one attack indicator as a function of at least said at least one index of concomitance between at least two different measured events, and detecting an attack attempt if said value of said at least one attack indicator crosses a predetermined threshold.
 11. The method of claim 10, wherein building said value of said at least one attack indicator comprises building said value out of several indices of concomitance between different measured events and weighting an importance of said indices of concomitance relative to one another using a predetermined set of weighting coefficients.
 12. The method of claim 11, further comprising building a plurality of attack indicator values through use of a plurality of different sets of weighting coefficients between said indices of concomitance, each set of weighting coefficients being predetermined so as to be more sensitive to an attack attempt that is different from attack attempts to which said other attack indicators are more sensitive
 13. The method of claim 12, wherein said weighting coefficient of a same index of concomitance is the same in all the sets of weighting coefficients used to build different attack indicator values.
 14. The method of claim 10, wherein said measurement of an event is limited to a sliding time slot so as to avoid taking account of events that have occurred outside said time slot.
 15. The method of claim 10, wherein at least one of said measured events is detection of an error in functioning of said security processor, each occurrence of which leads said security processor to stop processing operations in progress and to be automatically reset in order to restart said operations.
 16. The method of claim 10, wherein measurement of an event comprises counting, in a counter, a number of times said event has occurred, wherein a value of said counter constitutes a measurement.
 17. A manufacture comprising a tangible and non-transitory computer readable medium having encoded thereon software for enabling a security to detect an attempt to attack said security processor, said software comprising instructions for measuring events occurring independently of each other in absence of attack attempts; obtaining at least one index of concomitance between at least two measurements by multiplying the measurements with one another, the at least one index of concomitance representing a temporal proximity between at least two different measured events, building a value of at least one attack indicator as a function of at least said at least one index of concomitance between at least two different measured events, and detecting an attack attempt if said value of said at least one attack indicator crosses a predetermined threshold.
 18. An apparatus comprising a security processor, said security processor comprising registers in which are stored measurements of several events occurring independently of one another in absence of attack attempts, and a computer configured for obtaining at least one index of concomitance between at least two measurements by multiplying said measurements with one another, said measurements being stored in said registers, said at least one index of concomitance representing a temporal proximity between different measured events, building a value of at least one attack indicator as a function of said at least one index of concomitance between at least two different measured events, and detecting an attack attempt based on whether a value of said at least one attack indicator crosses a predetermined threshold. 